This invention relates to a method and apparatus for testing the ability of a substrate to transmit infrared radiation and is specifically directed to a method and apparatus for testing the ability of domes and windows of missiles and aircraft to transmit infrared radiation.
"Substrate" as used herein means the wall, or bulk material, which forms the dome or window and which has a first or outer surface and a second or inner surface and, while curved substrates are shown and discussed throughout, this invention will also test planar substrates.
The successful operation of a missile or an aircraft in identifying an object emitting infrared radiation depends upon the ability of its dome and/or window to transmit the received radiation therethrough and any loss in transmission in the substrate due to rain, sand and flying particles which damage the substrate and cause erosion, scratches, fractures and the like may increase the probability of failure of the missile or aircraft to perform its mission. Current requirements demand replacement of the dome or window if more than 4% in transmission loss has been detected.
Current methods of inspecting and testing these substrates for damage include a visual examination of the substrate surfaces or impinging the substrate surfaces with light and measuring the backscatter as a function of cracks or scratches on the inspected surfaces.
One problem with the current methods is the assumption that a fracture or a defect within the substrate will have a signature on the outer surface which can be detected. It has been found, however, that a fracture or defect within the substrate itself, ie, between the inner and outer surfaces, may not necessarily have a signature on the outer surface and the current methods will not detect such a fracture or defect.
Another problem with the current methods of testing substrates is that an undetected fracture or defect within the substrate may propogate to the surface of a substrate during flight operation and may cause a loss of transmission, or possibly the destruction of the dome or window, resulting in a failure of the missile or aircraft.
It is therefore an object of this invention to improve testing for transmission loss in a substrate by providing a means for testing not only the substrate surfaces for cracks, scratches, blemishes and the like but also within the substrate itself.
It is also another object of this invention to provide a method and apparatus for testing for transmission loss in a substrate and damage or defects within the substrate which may result in transmission loss or fracture and loss of the substrate resulting in loss of the airborne vehicle during flight.